Dedicated outdoor air system with pre-heating and method for same

ABSTRACT

A Dedicated Outdoor Air System (DOAS) includes a heater configured to be disposed within a supply air flow path, at least one pre-heater configured to be upstream from the heat exchanger within one or both of the supply and exhaust air flow paths, and a heat transfer device operatively connected to the heater and the pre-heater. The heat transfer device is configured to receive flue gas from the heater and transfer heat from the flue gas to liquid within the heat transfer device. The liquid is configured to be channeled to the pre-heater so that heat is transferred from the liquid to supply air within the supply air flow path.

BACKGROUND OF THE INVENTION

Embodiments generally relate to a system and method for pre-heating a dedicated outdoor air system (DOAS), and more particularly, to a high efficiency DOAS, gas heat exchanger or heater, and the like.

Enclosed structures, such as occupied buildings, factories and animal barns, and the like generally include an HVAC system for conditioning ventilated and/or recirculated air in the structure. The HVAC system includes a supply air flow path and a return and/or exhaust air flow path. The supply air flow path receives air, for example outside or ambient air, re-circulated air, or outside or ambient air mixed with re-circulated air, and channels and distributes the air into the enclosed structure. The air is conditioned by the HVAC system to provide a desired temperature and humidity of supply air discharged into the enclosed structure. The exhaust air flow path discharges air back to the environment outside the structure, or ambient air conditions outside the structure. Without energy recovery, conditioning the supply air typically requires a significant amount of auxiliary energy. This is especially true in environments having extreme outside air conditions that are much different than the required supply air temperature and humidity. Accordingly, energy exchange or recovery systems are typically used to recover energy from the exhaust air flow path. Energy recovered from air in the exhaust flow path is utilized to reduce the energy required to condition the supply air.

Conventional energy exchange systems may utilize energy recovery devices (for example, energy wheels and permeable plate exchangers) or heat exchange devices (for example, heat wheels, plate exchangers, heat-pipe exchangers and run-around heat exchangers) positioned in both the supply air flow path and the exhaust air flow path. A Dedicated Outdoor Air System (DOAS) conditions ambient/outside air to desired supply air conditions through a combination of heating, cooling, dehumidification, and/or humidification. In extremely cold conditions, however, frost may form on one or more energy recovery devices within a DOAS. For example, in extremely cold conditions, frost may form on an enthalpy wheel that first encounters outside air within the DOAS. Additionally, when a gas heater is added to a typical DOAS, the gas heater may only be 80% efficient. As such, there is a desire and need to increase the efficiency of such a system.

SUMMARY OF THE INVENTION

Certain embodiments of the present disclosure provide an energy exchange system that may include an energy recovery device configured to be disposed within supply and exhaust air flow paths, a heater configured to be disposed within the supply air flow path, wherein the heater is configured to generate flue gas, at least one pre-heater configured to be upstream from the energy recovery device within one or both of the supply and exhaust air flow paths, and a heat transfer device operatively connected to the heater and the at least one pre-heater. The heat transfer device is configured to receive energy from the flue gas from the heater and transfer heat from the flue gas to liquid within the heat transfer device. The liquid is configured to be channeled to the pre-heater(s) so that heat is transferred from the liquid to supply air within the supply air flow path before the supply air encounters the energy recovery device.

The system may also include pipes, tubes, conduits, or plenum connected between the heat transfer device and the heater. The flue gas is configured to pass from the heater to the heat transfer device via the one or more of pipes, tubes, conduits, or plenum.

The energy exchange system may be a Dedicated Outdoor Air System (DOAS). The energy recovery device may be one or more of an enthalpy wheel, a sensible wheel, a desiccant wheel, a plate heat exchanger, a plate energy exchanger, a heat pipe, or a run-around loop.

The system may also include a heat exchanger that includes the heater and at least one radiator coil configured to contain a heat transfer liquid. The radiator coil(s) may be configured to be disposed within or around a portion of the supply air flow path. The heat exchanger may be a liquid-to-gas heat exchanger. The heat exchanger may include one or more of a parallel flow heat exchanger, a counter flow heat exchanger, or a cross flow heat exchanger.

The pre-heater(s) may include a liquid-circulating coil in fluid communication with the heat transfer device. The liquid-circulating coil may be configured to be disposed within or around a portion of the supply air flow path.

The system may also include a liquid-circulating coil configured to be disposed within or around a flue gas passage. The liquid-circulating coil is configured to receive vented flue gas from the heater. The liquid-circulating coil is configured to be in fluid communication with the pre-heater.

The heater may be configured to be downstream from the energy recovery device within the supply air flow path. The heater may be configured to be upstream from the energy recovery device within the supply air flow path.

The system may also include at least one additional heat exchanger operatively connected to the heat transfer device.

The system may also include at least one return air duct configured to fluidly connect the supply air flow path with the exhaust air flow path.

The heat transfer device may be remote from the heater and the pre-heater. Optionally, the heat transfer device and the heater may be disposed within a common housing.

The system may include at least one bypass duct configured to be disposed within the supply air flow path. The at least one bypass duct may be configured to bypass at least a portion of the supply air around one or both of the pre-heater or the energy recovery device.

Certain embodiments of the present disclosure provide a method of operating a Dedicated Outdoor Air System having a supply air flow path that allows supply air to be supplied to an enclosed structure and an exhaust air flow path that allows exhaust air from the enclosed structure to be exhausted to the atmosphere. The method may include capturing flue gas generated by a heater, channeling the flue gas to a heat transfer device, transferring heat from the flue gas to liquid within the heat transfer device, circulating the liquid to a pre-heater, and transferring heat within the liquid to supply air within the supply air flow path.

The method may also include venting the flue gas from the heat transfer device after heat from the flue gas has been transferred to the liquid within the heat transfer device. The method may also include recirculating the liquid back to the heat transfer device after the heat within the liquid has been transferred to the supply air. The method may also include passing the heated supply air to an energy recovery device after the heat within the liquid has been transferred to the supply air. The method may also include preventing frost from forming on the energy recovery device through the channeling operation. The method may also include bypassing at least a portion of the supply air around one or both of the pre-heater or an energy recovery device.

Certain embodiments of the present disclosure provide a Dedicated Outdoor Air System (DOAS) that may include a heater configured to be disposed within a supply air flow path, a pre-heater configured to be upstream from the heater within the supply air flow path, and a heat transfer device operatively connected to the heater and the pre-heater. The heat transfer device is configured to receive flue gas from the heater and transfer heat from the flue gas to liquid within the heat transfer device. The liquid is configured to be channeled to the pre-heater so that heat is transferred from the liquid to supply air within the supply air flow path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic view of an energy exchange system, according to an embodiment.

FIG. 2 illustrates a schematic view of the energy recovery device, according to an embodiment.

FIG. 3 a illustrates a schematic view of a heat exchanger, according to an embodiment.

FIG. 3 b illustrates an isometric top view of an exemplary furnace, according to an embodiment.

FIG. 4 illustrates an isometric view of a radiator coil of a heat exchanger, according to an embodiment.

FIG. 5 illustrates an isometric view of a radiator coil of a heat exchanger, according to an embodiment.

FIG. 6 illustrates an isometric view of a radiator coil of a heat exchanger, according to an embodiment.

FIG. 7 illustrates an isometric view of a radiator coil of a heat exchanger, according to an embodiment.

FIG. 8 illustrates a schematic view of an energy recovery system, according to an embodiment.

FIG. 9 illustrates a schematic view of an energy recovery system, according to an embodiment.

FIG. 10 illustrates a schematic view of an energy recovery system, according to an embodiment.

FIG. 11 illustrates a process of operating a direct outdoor air system, according to an embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of certain embodiments will be better understood when read in conjunction with the appended drawings. As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.

FIG. 1 illustrates a schematic view of an energy exchange system 100 according to an embodiment. The system 100 is shown as a Dedicated Outdoor Air System (DOAS). The system 100 is configured to partly or fully condition air supplied to an enclosed structure 102, such as a building or an enclosed room. The system 100 includes an air inlet 104 fluidly connected to a supply air flow path 106. The supply air flow path 106 may channel supply air 108 (such as outside air, air from a building adjacent to the enclosed structure 102, or return air from a room within the enclosed structure 102) to the enclosed structure 102. Supply air 108 in the supply air flow path 106 may be moved through the supply air flow path 106 by a fan or fan array 110. The illustrated embodiment shows the fan 110 located downstream of an energy recovery device 112 and a gas-fired heater or heat exchanger 114. The heat exchanger 114 may be or include the gas-fired heater. Optionally, the fan 110 may be positioned upstream of the energy recovery device 112 and/or the heat exchanger 114. Also, alternatively, air 108 within the supply air flow path 106 may be moved by multiple fans or a fan array or before and/or after the heat exchanger 114.

Airflow passes from the inlet 104 through the supply air flow path 106 where the supply air 108 first encounters a pre-heater 116. A bypass duct 117 may be disposed in the supply air flow path 106. The bypass duct 117 may be connected to the supply air flow path 106 through an inlet damper 119 upstream from the pre-heater 116, and an outlet damper 121 downstream from the pre-heater 116. When the dampers 119 and 121 are fully opened, supply air 108 may be diverted or bypassed around the pre-heater 116. The dampers 117 and 121 may be modulated to allow a portion of the supply air 108 to bypass around the pre-heater 116.

Additionally, a damper 123 may be disposed in the supply air flow path 106 upstream from the pre-heater 116. When fully closed, the damper 123 prevents supply air 108 from passing into the pre-heater 116. The damper 123 may be modulated in order to allow a portion of the supply air 108 to a portion of the supply air 108 to pass through the pre-heater 116, while a remaining portion of the supply air 108 is bypassed through the bypass duct 117.

The pre-heater 116 heats the air 108 is it passes therethrough. The pre-heater 116 heats the incoming supply air 108 before it encounters a process side or portion of the energy recovery device 112. An additional pre-heater may be disposed within the supply air flow path 106 downstream from the pre-heater 116 and upstream from the energy recovery device 112. The additional pre-heater is configured to add more heat to the supply air 108 during extremely cold conditions. The pre-heater 116 may, alternatively, be disposed within the exhaust air flow path 120 upstream from the energy recovery device 120. Additionally, alternatively, a pre-heater may be disposed within the exhaust air flow path 120 upstream from the energy recovery device as well as the pre-heater 116 within the supply air flow path 106. As explained in more detail below with respect to FIG. 2, the energy recovery device 112 uses exhaust air 118 from an exhaust flow path 120 to condition the supply air 108 within the supply air flow path 106. For example, during a winter mode operation, the energy recovery device 112 may condition the supply air 108 within the supply air flow path 106 by adding heat and/or moisture. In a summer mode operation, the energy recovery device 112 may pre-condition the air 108 by removing heat and moisture from the air. An additional energy recovery device (not shown) may be positioned within the supply air flow path 106 downstream from the heat exchanger 114, and upstream from the enclosed structure 102. Additionally, while the energy recovery device 112 is shown upstream from the heat exchanger 114 within the supply air flow path 106, the energy recovery device 112 may, alternatively, be positioned downstream of the heat exchanger 114 and upstream of the enclosed structure 102 within the supply air flow path 106.

After the supply air 108 passes through the energy recovery device 112 in the supply air flow path 106, the supply air 108, which at this point has been conditioned, encounters the heat exchanger 114. The heat exchanger 114 then further or fully heats the air 108 in the supply air flow path 106 to generate a change in air temperature toward a desired supply state that is desired for supply air discharged into the enclosed structure 102. For example, during a winter mode operation, the heat exchanger 114 may further condition the pre-conditioned air by adding heat to the supply air 108 in the supply air flow path 106.

The exhaust or return air 118 from the enclosed structure 102 is channeled out of the enclosed structure 102, such as by way of exhaust fan 122 or fan array within the exhaust flow path 120. As shown, the exhaust fan 122 is located upstream of the energy recovery device 112 within the exhaust air flow path 120. However, the exhaust fan 122 may be downstream of the energy recovery device 112 within the exhaust air flow path 120.

The exhaust air 118 passes through a regeneration side or portion of the energy recovery device 112. The energy recovery device 112 is regenerated by the exhaust air 118 before conditioning the supply air 108 within the supply air flow path 106. After passing through the energy recovery device 112, the exhaust air 118 is vented to atmosphere through an air outlet 124.

In an alternative embodiment, additional bypass ducts and dampers may be disposed within the supply air flow path 106 and/or the exhaust air flow path 120 in order to bypass airflow around the energy recovery device 112.

FIG. 2 illustrates a schematic view of the energy recovery device 112, according to an embodiment. A portion of the energy recovery device 112 is disposed within the supply air flow path 106, while another portion of the energy recovery device 112 is disposed within the exhaust air flow path 120. The energy recovery device 112 is configured to transfer heat and/or moisture between the supply air flow path 106 and the exhaust air flow path 120. The energy recovery device 112 may be one or more of various types of energy recovery devices, such as, for example, an enthalpy wheel, a sensible wheel, a desiccant wheel, a plate heat exchanger, a plate energy (heat and moisture) exchanger, a heat pipe, a run-around loop, or the like. As shown in FIG. 2, the energy device 112 may be an enthalpy wheel.

An enthalpy wheel is a rotary air-to-air heat exchanger. As shown, supply air within the supply air flow path 106 passes in a direction counter-flow to the exhaust air within exhaust air flow path 120. For example, the supply air may flow through a lower portion, such as the lower half, of the wheel, while the exhaust air flows through an upper portion, such as the upper half, of the wheel. Alternatively, supply air may flow through a different portion of the wheel, such as a lower ⅓, ¼, ⅕, or the like, of the wheel, while exhaust air flows the remaining portion of the wheel. The wheel may be formed of a heat-conducting material with an optional desiccant coating.

In general, the wheel may be filled with an air permeable material resulting in a large surface area. The surface area is the medium for sensible energy transfer. As the wheel rotates between the supply and exhaust air flow paths 106 and 120, respectively, the wheel picks up heat energy and releases it into the colder air stream. Enthalpy exchange may be accomplished through the use of desiccants on an outer surface of the wheel. Desiccants transfer moisture through the process of adsorption, which is driven by the difference in the partial pressure of vapor within the opposing air streams.

Additionally, the rotational speed of the wheel also changes the amount of heat and moisture transferred. For example, an enthalpy wheel transfers both sensible and latent energy. The slower the rate of rotation, the less moisture is transferred.

The enthalpy wheel may include a circular honeycomb matrix of heat-absorbing material that is rotated within the supply and exhaust air flow paths 106 and 120, respectively. As the enthalpy wheel rotates, heat is picked up from the air within the exhaust air flow path 120 and transferred to the supply air within the supply air flow path 106. As such, waste heat energy from the air within the exhaust air flow path 120 is transferred to the matrix material and them from the matrix material to the supply air 108 within the supply air flow path 106, thereby raising the temperature of the supply air 108 by an amount proportional to the temperature differential between the air streams.

Optionally, the energy recovery device 112 may be a sensible wheel, a plate exchanger, a heat pipe, a run-around apparatus, a refrigeration loop having a condenser and evaporator, a chilled water coil, or the like.

Alternatively, the energy recovery device 112 may be a flat plate exchanger. A flat plate exchanger is generally a fixed plate that has no moving parts. The exchanger may include alternating layers of plates that are separated and sealed. Because the plates are generally solid and non-permeable, only sensible energy may be transferred. Optionally, the plates may be made from a selectively permeable material that allows for both sensible and latent energy transfer.

Alternatively, the energy recovery device 112 may be a run-around loop or coil. A run-around loop or coil includes two or more multi-row finned tube coils connected to each other by a pumped pipework circuit. The pipework is charged with a heat exchange fluid, typically water or glycol, which picks up heat from the exhaust air coil and transfers the heat to the supply air coil before returning again. Thus, heat from an exhaust air stream is transferred through the pipework coil to the circulating fluid, and then from the fluid through the pipework coil to the supply air stream.

Also, alternatively, the energy recovery device 112 may be a heat pipe. A heat pipe includes a sealed pipe or tube made of a material with a high thermal conductivity such as copper or aluminum at both hot and cold ends. A vacuum pump is used to remove all air from the empty heat pipe, and then the pipe is filled with a fraction of a percent by volume of coolant or refrigerant, such as water, ethanol, glycol etc. Heat pipes contain no mechanical moving parts. Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant.

Referring again, to FIG. 1, as supply air 108 enters the supply air flow path 106 through the inlet 104, the unconditioned supply air 108 encounters the pre-heater 116 before the energy recovery device 112, which may be an enthalpy wheel, flat plate exchanger, heat pipe, run-around, or the like, as discussed above. During winter months, when the air is cold and dry, the temperature and/or humidity of the supply air 108 will be raised as it moves through the pre-heater 116 and encounters the energy recovery device 112. As such, in winter conditions, the energy recovery device 112 warms and/or humidifies the supply air.

A similar process occurs as the exhaust air 118 encounters the energy recovery device 112 in the exhaust air flow path 120. The sensible and/or latent energy transferred to the energy recovery device 112 in the exhaust air flow path 120 is then used to pre-condition the air within the supply air flow path 106. Overall, the energy recovery device 112 pre-conditions the supply air 108 in the supply air flow path 106 before it encounters the heat exchanger 114, and alters the exhaust air 118 in the exhaust air flow path 120. In this manner, the heat exchanger 114 does not use as much energy as it normally would if the energy recovery device 112 (and/or the pre-heater 116) was not in place. Therefore, the heat exchanger 114 operates more efficiently.

The heat exchanger 114 may be or include a gas heater that coverts gas to heat, for example. Alternatively, the heater exchanger may be configured to transfer heat from liquid to air, for example. That is, the heat exchanger 114 may be a liquid-to-air heat exchanger. In general, the liquid and air are separated so that they do not mix. The heat exchanger 114 may include radiator coils that are positioned within or around the supply air flow path 106. Liquid, such as water or glycol, is circulated through the coils. As supply air 108 passes by the coils, heat from the liquid is transferred to the supply air 108, thereby further warming the supply air 108 before it passes into the enclosed structure 102. The radiator coils may be heated through combustion, for example, such as through a gas-fired heater. Heated gas from the heater is vented as flue gas. As explained below, the vented flue gas is channeled to the pre-heater 116 in order to pre-condition the supply air 108 before it encounters the energy recovery device 112. Alternatively, the heat exchanger 114 may not include radiator coils, but may simply be a gas heater disposed within the supply air flow path 106, and configured to convert gas to heat and heat the supply air 108.

In general, flue gas is a gaseous combustion product from a furnace or heating device. The flue gas may be formed primarily of nitrogen (for example, more than 2/3) derived from the combustion of air, carbon dioxide, and water vapor, as well as excess oxygen, which is also derived from the combustion of air.

FIG. 3 a illustrates a schematic view of the heat exchanger 114, according to an embodiment. As noted above, the heat exchanger 114 is disposed within the supply air flow path 106. The heat exchanger 114 includes a housing 126 that contains a gas-fired heater 128, such as a furnace, and radiator coils 130 that contain a liquid, such as water or glycol. The heater 128 may generate heat through combustion. The heater 128 heats the radiator coils 130 so that the liquid therein is heated. The radiator coils 130 are positioned within and/or around the portion of the supply air flow path 106 that passes through the heat exchanger 114. As supply air 108 passes through the heat exchanger 114, the temperature of the supply air 108 increases as it passes through the radiator coils 130. That is, the heat of the liquid within the radiator coils 130 is transferred to the supply air 108. Consequently, the temperature of the supply air 108 is increased as it passes out of the heat exchanger 114.

The flue gas from the heater 128 is vented through a vent 132 on or within the housing 126. The heat exchanger 114 may include a fan (not shown) that channels the flue gas into the vent 132. Optionally, the fan may be disposed downstream of the vent 132 within a conduit 134. The conduit 134 may be one or more pipes, tubes, plenum, or the like. For example, the conduit 134 may be a series of pipes that connect the vent 132 to another heat transfer device. The flue gas from the vent 132 then passes into the conduit 134 that sealingly engages the vent 132 so that the flue gas may be channeled to another heat transfer device, as described below.

Alternatively, the heat exchanger 114 may not include the radiator coils 130. Instead, the heat exchanger 114 may simply include the gas fired heater 128 disposed within the supply air flow path 106.

FIG. 3 b illustrates an isometric top view of an exemplary furnace 129, according to an embodiment. The furnace 129 is one example of a heater 128 (shown in FIG. 3 a). The furnace 129 includes a housing 127 having a plurality of heating elements 131 that span between lateral walls 133 of the housing 129. The heating elements 131 may include channeled rods having openings through which flames pass, thereby generating heat. The furnace 129 may be connected to a source of gas (not shown) that fuels the furnace 129. As gas enters the heating elements 131 and is ignited through an igniting element or pilot light within a control section 135, flames are generated. Additionally, flue gas is also generated from the heating elements. The temperature of the flame generated by the heating elements 131 may be approximately 2700° F., which generates a flue gas temperature of approximately 400° F. Various other furnaces may be used as the heater 128. FIG. 3 b merely shows one example of a furnace.

The heating elements 131 may include tubes that contain gas that is ignited to produce heat. The gas may make several passes through the tubes 131 before passing to the vent 132, shown in FIG. 3 a. As air within the supply air flow path 106 passes over the tubes 131, the air is heated.

FIG. 4 illustrates an isometric view of a radiator coil 130 a of the heat exchanger 114, according to an embodiment. As shown in FIG. 4, the radiator coil 130 a may be a tubular member 140 having a circumferential channel 142 surrounding an air passage 144, which may be a portion of the supply air flow path 106. Liquid, such as water or glycol, is circulated through the circumferential channel 142. In this manner, liquid L may flow parallel to the supply air 108 as it passes through the air passage 144. Optionally, the liquid may flow in a direction counter to the direction of the air flow (in this examples, the lines L would flow in the opposite direction than shown in FIG. 4).

FIG. 5 illustrates an isometric view of a radiator coil 130 b of the heat exchanger 114, according to an embodiment. In this embodiment, a plurality of tubes 146 having fluid channels 148 surround an air passage 150, which may be a portion of the supply air flow path 106. Liquid L, such as water or glycol, is circulated through the fluid channels 148. In this manner, liquid may flow parallel to the supply air 108 as it passes through the air passage 150. Optionally, the liquid may flow in a direction counter to the direction of the air flow.

FIG. 6 illustrates an isometric view of a radiator coil 130 c of the heat exchanger 114, according to an embodiment. In this embodiment, the radiator coil 130 c may include a series of fluid-filled plates 152 disposed within an air passage 154 that forms part of the supply air flow path 106. In this manner, the supply air 108 may flow across and parallel or counter to the liquid within the plates 152.

FIG. 7 illustrates an isometric view of a radiator coil 130 d of the heat exchanger 114, according to an embodiment. In this embodiment, the coil 130 d includes a plurality of liquid-carrying tubes 156 that cross one another. The tubes 156 are disposed within an air passage 158 that forms part of the supply air flow path 106. In this manner, the supply air 108 may flow across and parallel or counter to the liquid L within the tubes 156.

Any of the radiator coils shown and described with respect to FIGS. 4-7 may be used with respect to a heat transfer device in addition to, or in lieu of, the heat exchanger 114. For example, if the heat exchanger 114 does not include radiator coils, but instead simply includes a gas heater, the radiator coils may be used with respect to a heat transfer device, such as described below.

Additionally, referring to FIGS. 3 a-7, as noted, the heating elements 131 of the furnace 129 may include tubes that contain gas that is ignited to produce heat. Smaller tubes may be disposed within each of the tubes 131, similar to the configurations shown in FIGS. 4 and 5. For example, a main gas tube may surround a concentric liquid tube that contains heat transfer liquid. The liquid tube may be in fluid communication with the pre-heater 116, shown in FIG. 1. In this manner, the heat transfer liquid may be directly heated within the furnace and transferred to the pre-heater 116 to heat the supply air 106. As such, the temperature of the heat transfer liquid may be increased as it is directly heated within the furnace 129 and directly transferred to the pre-heater.

Referring to FIGS. 3 a-7, the heat exchanger 114 may be configured for parallel-flow, counter-flow, cross-flow, or a combination thereof. The radiator coil 130 shown in FIG. 3 a may be any of the radiator coils 130 a, 130 b, 130 c, or 130 d. In parallel flow, the supply air 108 and the liquid within the radiator coil 130 enter the heat exchanger 114 at the same end, and travel parallel to one another to the other side. In counter-flow, the supply air 108 enters at a front end of the heat exchanger 114, while the liquid enters the radiator coil 130 at the back end. In cross-flow, the supply air 108 and the liquid within the radiator coil are generally perpendicular to one another within the heat exchanger 114.

Referring to FIGS. 1 and 3 a, as noted above, flue gas from the heat exchanger 114 is vented to the conduit 134. The conduit 134 channels the flue gas to a heat transfer device 160, such as a heating coil, that may include an internal coil structure, similar to those described above. The heated flue gas passes through an internal chamber (not shown) of the heat transfer device 160. As the flue gas passes through the heat transfer device 160, the heat from the flue gas is transferred to the liquid within the radiator coils of the heat transfer device 160. The decreased-temperature flue gas (as heat from the flue gas has been transferred to the liquid) is then vented to the atmosphere through a vent 162. However, the liquid within the radiator coil of the heat transfer device 160, having an increased temperature through heat transfer with the flue gas, is channeled to the pre-heater 116 through a conduit 164. The heated liquid is then passed from the conduit 164 into an inlet 165 of a coil 166 of the pre-heater 116. The pre-heater 116 may also include radiator coils similar to that described above with respect to FIGS. 3 a-7. The liquid passed into the coil 166, the temperature of which has risen due to the heat transfer with the flue gas, then transfers the increased heat to supply air 108 that passes through the pre-heater 116. Accordingly, the supply air 108 is pre-heated (that is, the temperature of the supply air 108 is increased) before it encounters the energy recovery device 112.

As the liquid within the coil 166 circulates therethrough, the temperature of the liquid decreases, as its heat is transferred to the supply air 108. The cooled liquid within the radiator coil 166 passes out of the radiator coil 166 through an outlet 167 and into a conduit 168 that connects back to the heat transfer device 160. The liquid is then heated again by heat transfer with the flue gas, and the process repeats.

A pump 170 may be disposed within either of the conduits 164, 168, or both. The pump(s) 170 aids in circulating the liquid between the heat transfer device 160 to the pre-heater 116. However, in at least one embodiment, the system 100 does not include the pump.

While the pre-heater 116 and the heat transfer device 160 are described as including liquid-conveying coils, the pre-heater 116 and the heat transfer device 160 may be, or include, various other liquid-carrying and/or heating structures and components. For example, the pre-heater 116 may include fluid-conveying plates. Similarly, the heat transfer device 160 may be a heating plate(s). Additionally, each of the pre-heater 116 and the heat transfer device 160 may also include separate and distinct heating devices, similar to the heater 128 shown in FIG. 3. However, the liquid that is circulated between the heat transfer device 160 and the pre-heater 116 may be primarily or solely heated by way of heat transfer with the flue gas. Optionally, the liquid that is circulated between the heat transfer device 160 and the pre-heater 116 may be also heated through an electrical heater.

Additionally, while the heat transfer device 160 is shown as being separate, distinct, and remote from the heat exchanger 114 and the pre-heater 116, the heat transfer device 160 may be contained within a housing of the heat exchanger or the pre-heater 116. For example, the heat transfer device 160 may be mounted directly to the vent of the heat exchanger 114 inside or outside of the housing of the heat exchanger 114. As such, the heat exchanger 114 and the heat transfer device 160 may be disposed within a common housing.

Referring to FIGS. 1 and 3 a, the supply air 108 (that is, air supplied from outdoor and/or ambient air) is pre-heated by the pre-heater 116. The pre-heater 116 increases the temperature of the supply air 108 so that it will not form frost on the energy recovery device 112. The pre-heater 116 may increase the temperature of the supply air 108 through a circulating liquid that has been heated through a transfer of heat from harvested flue gas, as described above. As such, the efficiency of the system 100 is increased.

The following example illustrates the increased efficiency of the system 100. At point A, the supply air 108, which is outside air that enters through the inlet 104, flowing at a rate of 4000 cubic feet/minute (cfin), has a temperature of −20° F. However, the desired temperature within the enclosed space 102 at point B is 85° F., which represents a difference of 105° F. Therefore, the system 100 needs to raise the temperature of the outside air by 105° F. It has been found that the system 100, by way of harvesting the energy within the flue gas from the heat exchanger 114, is able to raise the temperature of the outside air to a temperature that exceeds the frost point. Accordingly, the system 100 that includes the pre-heater 116 and flue gas energy harvesting heat transfer device 160 renders frost control unnecessary, as the supply air 108 is raised to a temperature above the frost point before it encounters the energy recovery device 112. Accordingly, the energy recover device 112 does not need to be defrosted.

Moreover, the pre-heater 116 and the heat transfer device 160 may be retrofitted to any DOAS, thereby improving the efficiency of the DOAS.

FIG. 8 illustrates a schematic view of an energy recovery system 180, according to an embodiment. The system 180 is similar to the system 100, except that the heat exchanger 114 is upstream from the energy recovery device 112 within the supply air flow path 106. Therefore, the temperature of the supply air 108 is further heated after the pre-heater 116 before the supply air 108 encounters the energy recovery device 112. Thus, the possibility of frost forming on the energy recovery device 112 is further reduced. The system 180 may also include an additional heat exchanger downstream from the energy recovery device 112 within the supply air flow path 106.

FIG. 9 illustrates a schematic view of an energy recovery system 190, according to an embodiment. The energy recovery system 190 is similar to the system 100, except that an additional heat exchanger 192 is positioned upstream the energy recovery device 112. The heat exchanger 192 may be a liquid-to-gas heat exchanger. Flue gas from both the heat exchangers 114 and 192 is vented into a shared conduit 194 that channels the combined flue gas into the coil heater 160.

Additionally, in all of the embodiments, such as shown in FIGS. 1, 8, and 9, an optional return air duct may connect the exhaust air flow path 120 with the supply air flow path 106. For example, an air duct may be downstream of the energy recovery device 112 in the supply air flow path 106, and upstream of the energy recovery device 112 in the exhaust air flow path 120. Alternatively, or additionally, an additional return air duct may be upstream of the energy recovery device 112 in the supply air flow path 106 and downstream of the energy recovery device 112 within the exhaust air flow path 120. The return air ducts may recycle a portion of the exhaust air 118, which may be at a much higher temperature than outdoor air, into the supply air 108, which further increases the temperature of the supply air 108.

FIG. 10 illustrates a schematic view of an energy recovery system 200, according to an embodiment. The system 200 is similar to the system 100, except that return air ducts 202, 204, and 206 connect the exhaust air flow path 118 to the supply air flow path 106. More or less return air ducts than those shown may be used. Moreover, the return air ducts may be used with the systems 180 and 190 shown in FIGS. 8 and 9, respectively.

The return air duct 206 connects to the supply air flow path 106 upstream of the pre-heater 116. Thus, the temperature of the supply air 108 may be increased even before it encounters the pre-heater 116.

FIG. 11 illustrates a process of operating a direct outdoor air system, according to an embodiment. At 220, flue gas from a heat exchanger or heater is vented and captured within a conduit. The flue gas may be moved through the use of a fan, for example.

At 222, the flue gas is channeled to a heating device, such as a heating coil, plate, another heat exchanger, furnace, or the like. Next, at 224, the heat within the flue gas is transferred to liquid contained within the heating device. As the flue gas passes through the heating device and decreases in temperature (as the heat from the flue gas is transferred to the liquid within the heating device), the flue gas is vented from the heating device at 226. At the same time, at 228, the liquid, having an increased temperature due to heat transfer with the flue gas, is circulated to a pre-heater, which may include a liquid-circulating coil. Then, at 230, the heated liquid within the pre-heater is circulated around supply air flowing through a supply air flow path. Heat within the liquid is transferred to the supply air. At this time, the temperature of the liquid decreases, as a portion of its heat is transferred to the supply air. The liquid fully circulates through the pre-heater and is then recirculated back to the heating device at 232, and then the process returns to 224.

Additionally, flue gas and/or liquid may be bypassed to control the amount of energy transfer. Moreover, the flow of liquid may be modulated to control the amount of energy transfer.

Thus, embodiments provide a system and method of capturing heat energy from exhaust flue gas, and recycling the heat energy back into the supply air. Embodiments provide a system and method of using recycled flue gas energy to pre-heat an air stream to reduce the need for defrosting in cold conditions. Overall, embodiments provide a highly-efficient DOAS.

It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the various embodiments of the invention without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments of the invention, the embodiments are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.

This written description uses examples to disclose the various embodiments of the invention, including the best mode, and also to enable any person skilled in the art to practice the various embodiments of the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if the examples have structural elements that do not differ from the literal language of the claims, or if the examples include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

What is claimed is:
 1. An energy exchange system comprising: an energy recovery device configured to be disposed within supply and exhaust air flow paths; a heater configured to be disposed within the supply air flow path, wherein the heater is configured to generate flue gas; at least one pre-heater configured to be upstream from the energy recovery device within one or both of the supply and exhaust air flow paths; and a heat transfer device operatively connected to the heater and the at least one pre-heater, wherein the heat transfer device is configured to receive energy from the flue gas from the heater and transfer heat from the flue gas to liquid within the heat transfer device, and wherein the liquid is configured to be channeled to the at least one pre-heater so that heat is transferred from the liquid to supply air within the supply air flow path before the supply air encounters the energy recovery device.
 2. The system of claim 1, further comprising one or more of pipes, tubes, conduits, or plenum connected between the heat transfer device and the heater, wherein the flue gas is configured to pass from the heater to the heat transfer device via the one or more of pipes, tubes, conduits, or plenum.
 3. The system of claim 1, wherein the energy exchange system is a Dedicated Outdoor Air System (DOAS).
 4. The system of claim 1, wherein the energy recovery device is one or more of an enthalpy wheel, a sensible wheel, a desiccant wheel, a plate heat exchanger, a plate energy exchanger, a heat pipe, or a run-around loop.
 5. The system of claim 1, further comprising a heat exchanger, wherein the heat exchanger comprises the heater and at least one radiator coil configured to contain a heat transfer liquid, wherein the at least one radiator coil is configured to be disposed within or around a portion of the supply air flow path.
 6. The system of claim 5, wherein the heat exchanger is a liquid-to-gas heat exchanger.
 7. The system of claim 5, wherein the heat exchanger comprises one or more of a parallel flow heat exchanger, a counter flow heat exchanger, or a cross flow heat exchanger.
 8. The system of claim 1, wherein the at least one pre-heater comprises a liquid-circulating coil in fluid communication with the heat transfer device, wherein the liquid-circulating coil is configured to be disposed within or around a portion of the supply air flow path.
 9. The system of claim 1, further comprising a liquid-circulating coil configured to be disposed within or around a flue gas passage, wherein the liquid-circulating coil is configured to receive vented flue gas from the heater, and wherein the liquid-circulating coil is configured to be in fluid communication with the pre-heater.
 10. The system of claim 1, wherein the heater is configured to be downstream from the energy recovery device within the supply air flow path.
 11. The system of claim 1, wherein the heater is configured to be upstream from the energy recovery device within the supply air flow path.
 12. The system of claim 1, further comprising at least one additional heat exchanger operatively connected to the heat transfer device.
 13. The system of claim 1, further comprising at least one return air duct configured to fluidly connect the supply air flow path with the exhaust air flow path.
 14. The system of claim 1, wherein the heat transfer device is remote from the heater and the pre-heater.
 15. The system of claim 1, wherein energy from the flue gas prevents frost from forming on the energy recovery device.
 16. The system of claim 1, further comprising at least one bypass duct configured to be disposed within the supply air flow path, wherein the at least one bypass duct is configured to bypass at least a portion of the supply air around one or both of the pre-heater or the energy recovery device.
 17. The system of claim 1, wherein the heat transfer device and the heater are disposed within a common housing.
 18. A method of operating a Dedicated Outdoor Air System having a supply air flow path that allows supply air to be supplied to an enclosed structure and an exhaust air flow path that allows exhaust air from the enclosed structure to be exhausted to the atmosphere, the method comprising: capturing flue gas generated by a heater; channeling the flue gas to a heat transfer device; transferring heat from the flue gas to liquid within the heat transfer device; circulating the liquid to a pre-heater; and transferring heat within the liquid to supply air within the supply air flow path.
 19. The method of claim 18, further comprising venting the flue gas from the heat transfer device after heat from the flue gas has been transferred to the liquid within the heat transfer device.
 20. The method of claim 18, further comprising recirculating the liquid back to the heat transfer device after the heat within the liquid has been transferred to the supply air.
 21. The method of claim 18, further comprising passing the heated supply air to an energy recovery device after the heat within the liquid has been transferred to the supply air.
 22. The method of claim 21, further comprising preventing frost from forming on the energy recovery device through the channeling operation.
 23. The method of claim 18, further comprising bypassing at least a portion of the supply air around one or both of the pre-heater or an energy recovery device.
 24. A Dedicated Outdoor Air System (DOAS) comprising: a heater configured to be disposed within a supply air flow path; a pre-heater configured to be upstream from the heater within the supply air flow path; and a heat transfer device operatively connected to the heater and the pre-heater, wherein the heat transfer device is configured to receive flue gas from the heater and transfer heat from the flue gas to liquid within the heat transfer device, and wherein the liquid is configured to be channeled to the pre-heater so that heat is transferred from the liquid to supply air within the supply air flow path. 